There are two variants of this device. The REL21 has tungsten tipped electrodes
with a gap of 0.4mm. The electrodes are surrounded by a glass bead and
ceramic sleeves. The earlier CV86 was the Mk.I type and has electrodes surrounded
by a short piece of glass tubing to confine the glow discharge.

This device is gas filled with argon, plus a small amount
of mercury, and in Mk.II form can handle 120kW peak. These were used in the CHL (Chain
Home Low) coastal defence system in WWII, as well as the naval Type 291
air/surface warning radar.